Change requirements.txt to allow updating to latest patch version

Also, allow any version of pytz

requirements.txt

@@ -1,5 +1,5 @@
-Django==1.11.7
+Django>=1.11,<1.11.999
-djangorestframework==3.7.1
+djangorestframework>=3.7,<3.7.999
 drf-nested-routers==0.90.0
-pytz==2017.3
+pytz
 git+git://github.com/etesync/journal-manager@v0.4.1